Like most dope smokers, Jayne enjoyed the relaxed, mellow feeling she got after a 'session'. But at 19, she switched from having an occasional smoke to puffing as often as once an hour. She began to experience bizarre and sometimes frightening thoughts  even when she wasn't actually high.

"I thought I would die in my sleep," says Jayne, now in her twenties. "I thought that if I went to sleep, the people in my household would all die. So I was up, literally, [for] days, not wanting to sleep in case something happened to me or people I knew. I was up on a farm thinking that the dogs on the farm were looking at me directly. So I thought that I was a dog like them, and that was extremely scary... I really thought that these things were happening to me."

When people perceive the world in a disordered way, as Jayne did, they are said to have 'psychosis' or a 'psychotic episode'. This can involve hallucinations (seeing or hearing things that aren't real) and delusions (believing things that aren't true). Australia is a world leader in the prevention and early intervention of this condition, but people can still slip through the net.

When Jayne's psychosis got especially scary, she wound up in a hospital casualty ward, where she was sent home with some sleeping pills. But she eventually found the Melbourne-based Early
Psychosis Prevention and Intervention Centre (EPPIC), a specialist service for people between 15 and 30  which as it turns out, is the peak age range for both using cannabis (or marijuana),
and the onset of psychosis. This coincidence is just one reason why the association between cannabis and psychosis is so tricky to study.

Can cannabis cause psychosis?

EPPIC's figures show as many as 60 per cent of young people with early symptoms of psychosis used cannabis in the month before they came for treatment. Some of these patients go on to be diagnosed with schizophrenia (a more serious and longer-term mental illness, which incorporates other symptoms such as blunted emotions and social withdrawal).

So can using the 'evil weed' actually bring on a psychiatric illness that otherwise would not have occurred? Not according to the evidence, says Professor Wayne Hall, Director of the Office of Public Policy and Ethics at the Queensland University's Institute of Molecular Bioscience.

"If that were the case, we'd be expecting to see huge increases in numbers of people with schizophrenia and other psychoses over the last two decades, during which cannabis use has really exploded in Australia," says Hall. "So I think that's the least plausible [interpretation]."

But there is evidence that cannabis use, and particularly heavy cannabis use, can precipitate the disorder in people who are otherwise vulnerable, because of, say, a personal or family history of psychiatric symptoms. The strongest support for this idea comes from Sweden where researchers tracked the progress of over 50,000 young men recruited into the army. The research showed a statistically significant association between those who had smoked cannabis by age 18, and those who went on to develop schizophrenia.

"It was, in the jargon, 'a dose response relationship', meaning the more often you'd smoked cannabis by the age of 18, the higher the likelihood there was of developing schizophrenia," Hall says.

There is also good research evidence that people who continue to use the drug after the onset of their illness are more to likely to have more intense and frequent psychotic episodes.

Jayne fell into this category and at her lowest point, she rarely left the house, stopped washing her clothes, and ate poorly.

"If you didn't know I was psychotic and you popped in to visit me one day, you'd think I had given up on life and didn't care any more," she says.

Being diagnosed as mentally ill is sometimes enough of a shock to deter people from further drug use. But for those who continue, health workers take a 'harm minimisation' approach. EPICC's Kathryn Elkins says this might involve advising that weaker-strength cannabis be smoked and to not mix drugs, or discouraging the use of plastic fruit juice containers in home-made bongs. These can release toxic chemicals which have further harmful effects on the user. Support to encourage the development of psychological coping methods is also seen as very important.

"If they're using and they start to feel unwell, [we encourage them] to get help quickly, to talk to their case manager, to ring their support worker... to only use in groups with good friends, not to use when they're feeling down or depressed," Elkins says.

Cannabis and the brain

While one body of research links cannabis use with the development of schizophrenia in people who are more vulnerable to the illness, other work reveals a curious related finding. People with schizophrenia seem to be more likely to smoke cannabis  some reports indicate almost twice as likely.

Exactly why is not yet fully understood. One theory is that the drug may alleviate some of the side effects of schizophrenia medication. It may simply relieve feelings of boredom and unhappiness. But Dr Suresh Sundram, a psychiatrist from the Melbourne's Mental Health Research Institute, says: "Other people seem to use it simply because they've been introduced to cannabis use and it seems they are more likely to continue... [than] people without schizophrenia or other psychotic illnesses."

Dr Sundram and colleagues have been looking at how cannabis and its active ingredient, tetrahydrocannabinol (THC), might be working in the brains of people with schizophrenia. They found the brain tissue of those with the disease  whether cannabis users or not  have an increased number of certain chemical receptors (or target sites) for THC. Since the interaction between THC and its target sites is what initiates the drug's effects on the body, having more target sites might make people with schizophrenia inherently more susceptible to cannabis.

"This is the first direct evidence that this brain system might be altered in people with schizophrenia," Sundram says. "Smoking cannabis if you have schizophrenia, or if there's a strong predisposition towards schizophrenia or other psychotic illnesses, is not good for you... in fact cannabis is working on parts of your brain which are very closely related to those parts of the brain which may be responsible for psychosis."

Knowing the risks

Since the social stigma attached to mental illness is great, a confirmed diagnosis in a family member might not always be communicated to other relatives. This means many young people could be unaware of their individual vulnerability, and hence the risks of their drug-taking habit.

In Wayne Hall's view, rational public education on the issue has been a casualty of a wider debate about whether or not to change marijuana's illegal status. But legal or not, the drug's attraction remains.

"We've got two-thirds of young people using it now," Hall says. "They're using forms of the drug... where the THC content, for example, is often unknown. And they're using it in ways that are very risky and we're not doing a good job of educating [them] about the risks."

"In adolescence, anything more than about weekly use is probably getting into the territory where people may experience adverse consequences."

Since recovery from her psychosis, Jayne limits her cannabis use to a joint only once "every now and again".

"I think different temperaments are prone to perhaps having a psychosis, and unfortunately I'm one of those people," she says.

"I know that I can't go back to it being a habit and it never will be again. I'm pretty much of the attitude... if you're going to do it, do it, just be aware of the risks. And if your body can't handle it, figure that out as quickly as possible and stop it. It's definitely not right for everybody."

But the experience has brought new insights.

"Terrible as the whole thing was, it was definitely a big eye-opener on life in general, and that funnily enough boosted my confidence, and sort of got me into thinking... make the most of what you've got. So I've been really happy and on the way up ever since.